1. Field of the Invention
The present invention relates to a black image density correcting device for use in a full color copying machine capable of producing monochrome copies with black toner.
2. Description of the Prior Art
In a conventional color copying machine, and particularly, in a full color copying machine capable of reproducing original colors, the original is scanned three times to produce a full color copy. That is, in the first scanning, light reflected from the original document is passed through a blue filter to form a latent image on a photoconductor to which yellow toner, the complementary color of blue, is applied for development. In the second scanning, magenta toner is applied to a latent image formed on the photoconductor through a green filter, and finally in the third scanning, cyan toner is applied to a latent image formed through a red filter. The toner images of the above three colors are then transferred one on top of another onto a transfer medium disposed adjacent to the photoconductor, and the thus transferred image is further transferred onto a copy sheet to produce a full color image.
In recent years, full color copying machines provided with a black toner developer tank and capable of producing not only color copies but also monochrome copies with the black toner have come into common use.
In conventional full color copying machines capable of producing monochrome copies with black toner, toner density corrections for production of color copies are performed with respect to the toner transferred to the transfer medium. However, because of the need to electrically transfer toner images in register, carbon is dispersed in the film that forms the transfer medium belt, and therefore, the transfer medium has had a darkened surface of near black. As a result, since black toner is deposited on the black colored transfer medium when making monochrome copies with black toner, it has been difficult to detect the toner density on the transfer medium.
Therefore, in the prior art, the initial toner density has been set to a higher level to ensure production of stable images throughout the life of the developer, thereby eliminating the need for toner density corrections as the toner is consumed. However, this has caused degradation in the quality of initial images.